At first reading, because the author is so fixated by 50mm lenses, one gets the impression that there were no other cameras in existence until the Leica. All of those early press and folding cameras had lenses that were not 50 mm. Also, it was not just advertising and marketing. Schott Glass became a part of Zeiss to research and manufacture optical glass for the German optical industry, Abbe and others put the study and manufacture of lenses on a scientific industrial basis. It’s one thing to deign a lens that meets all specifications, but quite another to economically manufacture a lens with those desired specifications.
Probably one reason that the 50mm lens is so popular is that it is the most economical to manufacture and get quality results. Most fixed lenses are some variety of 50mm Tessar.
Most interchangeable lenses, especially for rf cameras, have the exact focal length somewhere on the lens to assist the camera mechanic during repairs. During assembly slight variations in focal length from that engraved always exist.
As for what is “normal” depends upon who is behind the lens. Also, the camera, subject, etc.
Just want to add that one of my late friends, who was a world renown photographer, worked almost, but not always, with a 50mm lens and a Leica. Some feel more comfortable with a 35mm or even a 28mm. Some like a 75mm. The only Hasselblad lens I owned for the longest time was a 60mm f4.
Having said all this, the piece was well crafted, although I would have preferred that Professor Abbe be given credit for developing optical theory regarding microscope lenses. Three remarkable men: Abbe, Zeiss and Schott, who rejected the inheritance of wealth by setting up the Carl Zeiss Stifftung for the advancement of optical science.